1. Field of the Invention
This invention relates generally to optical telecommunication systems and, more particularly, to transmitters comprising photonic integrated circuits employed in such systems.
2. Description of the Related Art
Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals, each having a different wavelength, are combined onto a single optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the output of the laser, and an optical combiner to combine each of the modulated outputs.
Conventionally, WDM systems have been constructed from discrete components. For example, the lasers, modulators and combiners have be packaged separately and provided on a printed circuit board. More recently, however, many WDM components have been integrated onto a single chip, also referred to a photonic integrated circuit (PIC).
In order to further increase the data rates associated with WDM systems, various modulation formats have been proposed for generating the modulated laser output.
One such modulation format, known as polarization multiplexed differential quadrature phase-shift keying (“Pol Mux DQPSK”), can provide higher data rates than other modulation formats, such as an amplitude modulation format. A transmitter outputting Pol Mux DQPSK signals, however, typically has more components, both active and passive, and greater complexity than an amplitude modulating transmitter. Optical signals propagating through or generated by the various PIC components may be subject to loss, noise, and signal distortion (collectively, “signal degradations”), which increase with increased PIC integration. Moreover, due to processing variations during fabrication of the PIC, signal degradations may not be uniform for each optical signal. For example, certain components on the PIC may induce more loss than others.
What is needed is a photonic integrated circuit having reduced signal degradations.